1. Field of the Invention
This invention relates to teletext decoders for receiving, storing and processing teletext information which is transmitted as digitally coded data and comprises a plurality of different pages each of which is identified by a respective page number. Current transmissions of teletext information are in television signals in television lines where no picture signals representing normal picture information are present.
2. Description of the Prior Art
The document "Broadcast Teletext Specification", September 1976, published jointly by the British Broadcasting Corporation, Independent Broadcasting Authority and British Radio Equipment Manufacturers' Association, discloses a specification for a television transmission of the above type for use in 625-line television systems.
In the above-identified document "Broadcast Teletext Specification", a quantity of teletext information to be considered as an entity is termed a page and will be so termed hereinafter. All of thepages which are available are normally transmitted in a recurrent cycle, with or without up-dating page information, as appropriate. At a teletext decoder any page can be selected, and the digitally coded data representing the page information is then acquired by the teletext decoder from the cyclic transmission and is stored in a page memory of the teletext decoder for as long as the page is required. The pages are organised into different magazines and each page consists of up to 24 data rows each having 40 character positions. The first data row (Row 0) of each page is termed a page-header and contains inter-alia the page number. The transmission of each page begins with, and includes, its page-header and ends with, and excludes, the next page-header which is transmitted in respect of a page in the same magazine. Thus, it is assumed that all of the data rows containing the relevant magazine number which are transmitted between two such successively transmitted different page-headers belong to the page having the first page-header. The assumption avoids the need to include in each data row data for identifying the page to which the row belongs. Also, there is row-adaptive transmission in that data rows containing no information are not transmitted. Because different groups (magazines) of pages use the same range of page numbers, so that more than one page can have the same page number within that range, and bacause, as transmitted, the pages from different magazines may have their data rows interleaved, each data row also includes data which identifies the magazine containing the page that the row belongs to.
It is also a requirement of said "Broadcast Teletext Specification" that the data for an entire data row is contained in a single television line that is used for transmitting teletext information. Such a television line is termed a data-line and will be so termed hereinafter. The "Broadcast Teletext Specification" further specifies that only data-lines in the vertical or field-blanking intervals (VBI) should be used for transmitting teletext information. Initially, teletext transmissions in the United Kingdom used television lines 17 and 18 in odd fields and television lines 330 and 331 in even fields as data-lines, but current teletext transmissions in the United Kingdom now use six television lines in each field as data-lines. The VBI transmission is thus, in effect, multiplexed with the normal picture transmission and the teletext information transmitted in this way and representing data for display such as text and graphics can be displayed at a teletext television receiver as a selectable alternative to the normal picture information.
The said "Broadcast Teletext Specification" provides a page erasure interval by specifying, in effect, that rows of a page will be transmitted such as to allow an active television field period between the transmission of a page-header of a page and the transmission of subsequent data rows for the page. This interval, which is 20 ms. in 625-line television systems, allows time for a teletext decoder to respond to receipt of the page-header of a new page selected by a viewer to erase from its page memory any previously acquired page that is already stored therein, before the data rows of the new page are received. The page memory is thus made ready to store the data rows of thenew page as transmitted in the data-lines of field-blanking intervals subsequent to the field-blanking interval in which the page-header was transmitted. This provision simplifies teletext decoder design by allowing the operation of the teletext decoder to be relatively slow.
If the data of an `old` page in the page memory is not erased before a `new` page is stored therein, the resulting total data would include any data rows of the `old` page that have not been over-written by corresponding data rows of the `new` page. In other words, one or more spurious data rows would occur in every stored `new` page that did not contain the full 24 data rows.
A proposed extension of teletext transmission is for "full-channel" operation in which all of the television lines of each field of a TV channel are employed for transmitting teletext information. Thus, a complete TV channel, which can be an over-air broadcast channel or a cable channel, is dedicated to teletext transmission.
If a teletext specification adopted for "full-channel" operation included the 20 ms. page erasure interval discussed above, then after the transmission of a page-header of a particular page in one television (data) line, 312 subsequent data-lines would have to occur before the first data row of that page could be transmitted. This means that the flexibility and usable capacity of a TV channel dedicated to "full-channel" operation would be severely restricted. For instance, for a serial mode of page transmission in which the pages are transmitted sequentially with all the data rows of each page being transmitted successively between the page-header for the page and the page-header for the next page, only about 10% of the total capacity of the channel would be usable.
Therefore, for efficient "full-channel" operation, a teletext transmission is required in which data rows of a page can be transmitted immediately after the transmission of the page-header of the page. In other words, in the most extreme case, the first data row of a page should be transmitted in the next data-line following the data-line in which the page-header for the page is transmitted. However, this reduces an effective minimum page erasure interval for a teletext decoder to approximately 40 .mu.s, (i.e. the period in a data-line that contains the page-header data) and this poses the problem of making the teletext decoder operate fast enough after recognition of a selected page in one data-line to clear its page memory in time to store the first data row of the selected page as received in the immediately following data line.
Patent specification GB No. 2 115 258 A discloses an image display apparatus for the Japanese multi-character broadcast or CAPTAIN system which can be characterised as a teletext system in which information is transmitted utilising the vertical blanking period of a television broadcast. This patent specification is directed to the problem of fast erasure of a page of data in a page memory so that the page memory becomes available very quickly for storing the data of a new page. The solution to this problem involves the detection by the image display apparatus of an erasing code which is sometimes included in a page control packet (analogous to a page-header). When the erasing code is detected in the control packet for a `new` page, a flag is set in the apparatus for each portion (row or dot line) of an `old` page to be erased from the page memory. These flags are then reset progressively as the data for the `new` page is received in subsequent television fields. Whilst a flag is set, `blanks` are written for display instead of the portion of the `old` page to which the flag pertains. Once a flag has been reset the appertaining portion of the `new` page is written for display. The effect of this solution is that the page memory does not have to have the data of the `old` page emptied from it before it can start receiving the data of the `new` page.